Biosynthesis of 32P-labelled hydroxocobalamin and a study of its behaviour in rats

Using Propionibacterium freudenreichii and 32P-ATP, batches of 32P-labelled cobalamin (Cbl) were biosynthesized with a maximum specific activity of 61 microCi/mg, i.e. about 100 times higher than previously reported. Pharmacological doses mixed with 57Co-Cbl were injected subcutaneously in the form of hydroxo-Cbl into rats subsequently killed 5-20 days later. The two labelled Cbls were distributed in approximately the same way, the highest concentration being found in kidney (typical for rats) and about one-fifth of that in liver. These findings tallied with previous observations with radioactive cyano-Cbl and microbiological assay. In all injected rats, the 57Co/32P ratio was lower in liver than in kidney. Drugs eradicating the intestinal flora had no influence. In rats receiving the vitamin orally, the ratio was higher in liver than in kidney. All of our findings could be due to formation of a cobinamide-like compound lacking phosphorus. It is concluded that we have produced radiophosphorus-labelled Cbl that enables studies in vivo.